The present invention generally relates to mobile communication terminals and transmission power control methods therefor, and more particularly to a mobile communication terminal which guarantees stable operation even when the mobile communication terminal moves at a high speed, and to a transmission power control method applicable to such a mobile communication terminal.
In mobile communication systems, there are demands to develop a system having a high frequency utilization efficiency. Among various systems proposed, a direct sequence code division multiple access (DS-CDMA) is regarded as the most prominent system that can improve the communication capacity.
When the DS-CDMA is applied to the mobile communication, received levels of up-link signals transmitted from a plurality of mobile communication terminals must be approximately the same at a base station. Otherwise, signal interference rates of the up-links from each of the mobile communication terminals will not become approximately the same, and it will be impossible to reproduce the up-links from the mobile communication terminals due to the poor signal interference rates.
For this reason, in the mobile communication employing the DC-CDMA, it is essential to control the reception power of each mobile communication terminal at a high speed with a high accuracy and a large dynamic range depending on changes in the distance from each mobile communication terminal to the base station, the shadowing of each mobile communication terminal caused by buildings or the like, and instantaneous fluctuations in the multipath of each mobile communication terminal.
The changes in the distance from the mobile communication terminal to the base station and the shadowing similarly occur on the up-links having different frequency bands. Hence, it is possible to cope with the changes in the distance from the mobile communication to the base station and the shadowing, by carrying out an open loop control which controls the transmission level of the up-link on which the mobile communication terminal transmits depending on the reception level of the down-link received by the mobile communication terminal. However, it is only possible to cope with the instantaneous fluctuations in the multipath by a closed loop control because the correlation between the up-link and the down-link is low when the frequency bands greatly differ. The closed loop control controls the transmission level of the up-link on which the mobile communication terminal transmits by sending a control command from the base station to the mobile communication terminal depending on the reception level of the up-link received by the base station.
According to the conventional DS-CDMA mobile communication system, a receiver of the base station detects the reception level from the mobile communication terminal which is to be controlled, and the instantaneous SIR and bit error rate (BER) are inferred. In addition, based on the above reception level, SIR and BER, a control command for controlling the transmission power of the mobile communication terminal is generated and transmitted to the mobile communication terminal with the down signal. The transmission rate of the control command for controlling the transmission power must be high enough to follow the instantaneous fluctuations in the reception level, SIR or BER generated on the up-link. For example, according to the standard IS-95, the control command rate is 800 bps, and an amount of control per command is 0.5 dB.
When the mobile communication terminal is located in a vehicle and the mobile communication terminal moves at a high speed, the fluctuating rate of the instantaneous fluctuations (Rayleigh fluctuations) of the multipath also becomes high, and the fluctuation width per unit time becomes large. In order to obtain a desired BER in such a case, the rate of the control command for controlling the transmission power of the mobile communication terminal must be made high compared to that of a slowly moving mobile communication terminal. However, when the control command rate is set high, the ratio of the control command with respect to the entire communication capacity of the down-link becomes large, and there is a problem in that the usable communication capacity becomes small.
Accordingly, it is a general object of the present invention to provide a novel and useful mobile communication terminal and transmission power control method therefor, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a mobile communication terminal in which the transmission power is controlled so that a reception state on an up-link of a base station becomes approximately constant without being affected by instantaneous fluctuations even when the mobile communication terminal moves at a high speed, without having to increase the transmission rate of a control command that is transmitted from the base station to the mobile communication terminal, and to a transmission power control method for use in such a mobile communication terminal.
Still another object of the present invention is to provide a mobile communication terminal which receives a control command transmitted on a down-link from a base station and controls a transmission power so that a reception state on an up-link becomes approximately constant at the base station, which mobile communication terminal comprises a moving speed inferring unit inferring a moving speed of the mobile communication terminal, and a transmission power controller varying a varying width of the transmission power depending on the moving speed inferred in the moving speed inferring unit. According to the mobile communication terminal of the present invention, it is possible to make the varying width of the transmission power larger as the moving speed becomes faster and the instantaneous fluctuation becomes faster. For this reason, it is possible to make the reception state on the up-link of the base station approximately constant, without the need to increase the transmission rate of the control command of the base station. It is also possible to prevent the communication capacity of the down-link from decreasing. Furthermore, since the basic structure of the base station does not require modification, it is possible to prevent the cost of the system from increasing.
A further object of the present invention is to provide a transmission power control method adapted to a mobile communication terminal which receives a control command transmitted on a down-link from a base station and controls a transmission power so that a reception state on an up-link becomes approximately constant at the base station, where the mobile communication terminal includes a moving speed inferring unit inferring a moving speed of the mobile communication terminal, and a transmission power controller varying a varying width of the transmission power depending on the moving speed inferred in the moving speed inferring unit, and the transmission power control method comprises the steps of (a) employing a direct sequence code division multiple access (DS-CDMA) on the up-link and the down-link and transmitting a pilot signal on the up-link using an extrapolation technique, (b) varying, in the transmission power controller, a ratio of transmission powers of a pilot transmission signal and an information data transmission signal on the up-link depending on the moving speed inferred by the moving speed inferring unit, and (c) accumulating, in the base station, control commands to be transmitted to the mobile communication terminal and varying a passing bandwidth of a filter which is provided with respect to a reverse spread pilot signal on the up-link depending on an accumulated value. According to the transmission power control method of the present invention, it is possible to set the transmission power of the pilot transmission signal on the up-link to a minimum required value depending on the moving speed of the mobile communication terminal. In addition, it is possible to vary the passing bandwidth of the reverse spread pilot signal on the up-link depending on the power variation of the pilot signal on the up-link in accordance with the moving speed of the mobile communication terminal. Therefore, it is possible to reproduce the pilot signal in an optimum state.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.